1. Field of the Invention
The present invention relates to a workpiece clamping unit driving mechanism for a sewing machine and, more specifically, to a workpiece clamping unit driving mechanism for a bar tacking machine, for moving a workpiece clamping unit clamping a workpiece in parallel to the upper surface of the bed of the bar tacking machine in predetermined directions for stitching operation.
2. Description of the Related Art
Workpiece clamping unit driving mechanisms of the foregoing kind are disclosed in, for example, JP-A No. 8-84877 and JP-B Nos. 60-27307 and 60-17548.
A workpiece clamping unit driving mechanism disclosed in JP-A No. 8-84877 is employed in a sewing machine comprising a workpiece clamping unit, and a workpiece carrying unit for carrying the workpiece clamping unit along the surface of a bed included in the sewing machine to change the position of the workpiece relative to a needle. The workpiece carrying unit comprises a motor disposed on the lower surface of the bed with its output shaft extended in parallel to a bed shaft included in the sewing machine, and a driving force transmitting mechanism for converting the output torque of the motor into a linear force acting in parallel to the surface of the bed and transmitting the linear force to the workpiece clamping unit. The driving force transmitting mechanism comprises a helical driving member coaxially fixed to the output shaft of the motor and provided with a helical groove in the circumference thereof, a driven member engaged with the helical groove so as to be moved along the axis of the output shaft of the motor when the helical driving member is rotated, and a transmission mechanism for converting the movement of the driven member along the axis of the output shaft of the motor into a movement parallel to the surface of the bed of the sewing machine and transmitting the movement parallel to the surface of the bed of the sewing machine to the workpiece clamping unit.
This prior art workpiece clamping unit driving mechanism is intended to use a cylindrical cam, a worm or the like as the helical driving member, and a cylindrical roller, a worm wheel or the like as the driven member. Therefore, this prior art workpiece clamping unit driving mechanism has technical problems that the motor must be disposed with its output shaft in parallel to the bed shaft of the sewing machine, has a complicated construction and is difficult to manufacture. Particularly, it is difficult to engage the helical driving member and the driven member so that the driven member is fitted closely in the groove of the helical driving member and low friction acts between the helical driving means and the driven member, the workpiece clamping unit driving mechanism is liable to rattle due to gaps between the driven member and the walls defining the groove of the helical driving member in which the driven member is fitted, and hence it is very difficult to move the workpiece clamping unit accurately along the surface of the bed. Moreover, the helical driving member or the driven member needs to be changed soon to stop play between the helical driving member and the driven member, and the change of the helical driving member or the driven member requires troublesome work.
Suppose, for example, that a cylindrical cam 91 and a roller 93 shown in FIG. 8 are used as the helical driving member and the driven member, respectively, the roller 93 is fitted in a cam groove formed in the circumference of the cylindrical cam 91 and is in engagement with the side surfaces 91a of the cam groove. If there is any gap between the side surface 91a of the cam groove and the roller 93, there will be a lost motion in a driven lever 92 holding the roller 93 relative to the movement of the cylindrical cam 91 and, consequently, the workpiece clamping unit of a workpiece clamping mechanism is unable to carry the workpiece correctly and hence it is difficult to position the workpiece correctly. Consequently, the quality of bar tacking stitches is deteriorated.
Since the driven lever 92 holding the roller 93 turns about an axis C92, the distance between the axis C93 of the roller 93 and the axis C91 of the cylindrical cam 91 varies according to the variation of the angular displacement of the driven lever 92. Therefore, the side surfaces 91a of the cam groove must be complicated curved surfaces and a special machining apparatus is necessary to form the cam groove; that is, the cam groove must be formed by an eccentric cutting apparatus capable of displacing the center C95 of a cutting tool 95 in a direction perpendicular to the axis C91 of the cylindrical cam 91 according to the rotation of the cylindrical cam to make the center C95 of the cutting tool 95 coincide with the center C93 of the roller 93 which turns about the center axis C92. Since the cylindrical cam 91 and the roller 93 engaged with the cylinder cam 91 must be formed in a high accuracy, it is more difficult to form the cylindrical cam 91 than to form a general plane cam, and the use of the combination of the cylindrical cam 91 and the roller 93 increases the manufacturing cost of the bar tacking machine.
Since the roller 93 held by the driven lever 92 and engaged with the cylindrical cam 91 slides along the circumference of the cylindrical cam 91 and move along the axis of the cylindrical cam 91 as the cylindrical cam 91 is rotated, the roller 93 needs to slide against sliding friction through a long distance, so that the side surfaces 91a of the cam groove, and the roller 93 are abraded rapidly. Since the roller 93 moves in the cam groove defined by the opposite side surfaces 91a, it is difficult to achieve simultaneously both the smooth rotation of the roller 93 and the elimination of the play of the roller 93 in the cam groove defined by the side surfaces 91a, and the sliding friction between the side surfaces 91a of the cam groove and the roller 93 is inevitably high. There is no means for eliminating the play of the movement of the workpiece clamping mechanism in X- and Y-directions resulting from the abrasion of the side surfaces 91a of the cam groove or the roller 93 held by the driven lever 92 other than a means which changes the cylindrical cam 91 or the roller 93 held by the driven lever 92, which increases maintenance costs.
A workpiece clamping unit driving mechanism for a sewing machine disclosed in JP-B No. 60-17548 comprises a motor disposed above the bed of the sewing machine, a motor disposed inside the bed, timing pulleys fixedly mounted on the output shafts of the motors, respectively, timing belts driven by the timing belt pulleys, an X-axis block fixed to the X-axis timing belt, a Y-axis block fixed to the Y-axis timing belt, an X-axis guide rail for guiding the X-axis block, and a Y-axis guide rail for guiding the Y-axis block, fixed to the X-axis block. The X-axis block is moved along the X-axis guide rail, the Y-axis block is moved along the Y-axis guide rail to move the workpiece clamping unit fixed to the Y-axis block in X- and Y-directions.
If the workpiece clamping unit driving mechanism has large play owing to the elongation of the timing belts or backlashes between the timing belts and the corresponding timing belt pulleys, the workpiece cannot be accurately fed, the workpiece cannot be accurately positioned, and thereby the quality of bar tacking stitches is deteriorated. Moreover, the mechanism for guiding the workpiece clamping unit in the X- and the Y-direction is complicated, needs precision parts to drive the workpiece clamping unit smoothly and hence needs a high manufacturing cost.